1. Field of Invention
When a negative feedback control system such as a phase lock loop is not in a locked condition, some kind of sweep such as a frequency sweep over a predetermined search range is used to attempt to find the appropriate parameter such as frequency that is necessary to establish locked conditions. This would be the carrier frequency or clock that is presently being used by the data supplied to a phase lock loop. When a lock condition is established, it is imperative that the sweep be timely disabled so as to prevent the system from immediately losing lock again.
2. Description of Related Art
Known prior art approaches to detecting frequency acquisition have tried 1) embedding a low frequency oscillator in the phase lock loop and 2) detecting the difference frequency "beat note" to determine the lock status of the phase lock loop. The approach used for the first technique referenced supra required the modification of the transfer function of the phase lock loop. This modification, however, meant that it was sensitive to the level of the data pattern dependent phase jitter or, in other words, of random phase perturbations. The presence of non-linear elements (i.e., diodes) causes the out-of-lock detection to be sensitive to the level of the data pattern jitter or momentary phase perturbations. Because of this, the loss of lock detection circuit would think that it had lost "lock" under many conditions when it had not. The second prior art technique, when used with a wide tuning range (greater than one megahertz), caused the loss of lock detection circuit to be sensitive to data pattern jitter thereby initiating false frequency sweeping. This is due to the wide band note detection circuitry mistaking the data pattern energy for a beat note. This second technique is capable of detecting false phase lock but must be restricted to a single direction frequency approach to the lock frequency to overcome false locking.